1. Field of the Invention
The present invention relates to a method and an apparatus for vaporizing a liquid raw material into an ingredient gas and feeding this ingredient gas as a reacting gas into a reactor for epitaxial growth.
2. Description of the Related Art
As a reacting gas fed into this type of reactor for epitaxial growth, there is mainly used a gas obtained by vaporizing a liquid raw material such as dichlorosilane (SiH.sub.2 Cl.sub.2), trichlorosilane (SiHCl.sub.3) or tetrachlorosilane (SiCl.sub.4). These raw materials except dichlorosilane are liquids at room temperature and atmosphere pressure.
Hitherto, the reacting gas as an ingredient gas has been mixed with a carrier gas to feed the mixed gas into a reactor. As illustrating in, for example, FIG. 4, the method for feeding this mixed gas includes a method of blowing a carrier gas 3 into a liquid raw material 2 to bubble the liquid raw material 2, thereby producing a mixed gas 4 of an ingredient gas obtained by vaporizing the liquid raw material and the carrier gas 3, and feeding the mixed gas, as it is, into a reactor for epitaxial growth 6. Other methods include a method of further mixing the mixed gas with diluting hydrogen gas to obtain a mixed gas whose ingredient gas has a predetermined concentration, and feeding the mixed gas into a reactor, as not illustrated in any figures, and a method of further injecting a dopant such as phosphorus into the mixed gas and feeding the resultant gas into a reactor. The mixed gas is fed into the reactor in such a manner as above, so as to epitaxial-grow a silicon mono-crystal thin layer on a mono-crystal silicon substrate set in the reactor.
The above-mentioned methods of feeding the mixed gas by bubbling have the following problems.
1) In the case of using a gas cylinder having a volume for filling up a 25 kg liquid raw material and the total weight of about 50 kg, considering handling by a worker, and feeding the mixed gas into plural reactors with the single gas cylinder, the concentration of the ingredient gas included in the mixed gas is easily changed, so that the respective reaction speeds in the reactors are changed. Therefore, the mixed gas can be in general fed from the single gas cylinder to only one reactor.
2) The flowing amount of the ingredient gas depends on the vapor pressure of the liquid raw material which is changed dependently on the temperature of the liquid, the pressure in the gas cylinder, and the flowing amount of the carrier gas. As a result, controlling the concentration of the ingredient gas is complicated.
3) When the liquid raw material is consumed so that the remaining amount thereof in the gas cylinder is decreased, the period for bringing the gas produced by bubbling in contact with the liquid raw material is shortened and the temperature of the liquid raw material is lowered by latent heat of the liquid raw material evaporated in bubbling. Thus, the concentration of the produced ingredient gas is reduced. As a result, the reaction speed in the reactor is also reduced.
4) Because the step for producing the ingredient gas from the liquid raw material by bubbling is a sort of distillation step, a very small amount of heavy metals and high boiling point impurities included in the liquid raw material remain in the liquid. Thus, as evaporation of the liquid raw material advances, the concentration of the impurities therein relatively rises. As a result, the amount of the impurities in the ingredient gas obtained by bubbling increases as the amount of the liquid decreases.
5) Whenever the gas cylinder is exchanged, the mouth for connecting the gas cylinder to the reactor is opened to the atmosphere. Thus, at that time substances such as water in the atmosphere come into the gas feeding system to deteriorate the quality of epitaxially grown thin layer.
6) Whenever the gas cylinder is exchanged, it is necessary to make a trial for checking the reaction condition for epitaxial growth in the reactor.